<!DOCTYPE html>
<meta charset="utf-8">
<title>reference: 0.8 opacity green circle</title>
<style>

body {
  margin: 0;
}

div {
  margin: 100px 200px;
  width: 200px;
  height: 200px;
  background-image: url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" viewbox="0 0 200 200"><circle cx="100" cy="100" r="100" fill="green" fill-opacity="0.8"/></svg>');
}

</style>

<div></div>
